Processes for hydroformylating an olefin to prepare a carbonyl derivative containing one carbon atom more than the parent olefin by reacting the olefin with synthesis gas in the presence of a rhodium catalyst in complex combination with carbon monoxide and a triorganophosphorus ligand (e.g., triphenylphosphine) are well known in the art and of growing industrial importance. This technology is summarized, for example, in U.S. Pat. No. 3,527,809 issued Sept. 8, 1970 to Pruett et al. The olefin reactant, together with a gaseous mixture of carbon monoxide and hydrogen, is typically bubbled through a liquid reaction medium, which may or may not comprise a separate inert liquid solvent species, which contains as hydroformylation catalyst a complex of rhodium with carbon monoxide and a triorganophosphorus ligand, the ligand typically being present in some excess. It will be understood also that hydrogen will be present in the complex during a portion of the catalytic cycle. The carbonyl reaction product is removed from the reaction medium, either by being stripped directly out of the reaction zone in the stream of unreacted synthesis gas exiting from the reaction zone or else by being distilled out of a liquid draw-off stream which is continuously withdrawn from the reaction zone for product recovery. Both techniques may be employed simultaneously, of course.
It is known that some deactivation of the rhodium-containing catalyst takes place with the passage of time, necessitating periodic withdrawal of at least a portion of the liquid contents of the reaction zone for recovery of deactivated rhodium catalyst and reconversion into an active form. This deactivation is, in these systems, a more than ordinarily significant cost factor inasmuch as rhodium, even in very small quantities, is expensive. It is therefore recognized in the art that preventing or minimizing deactivation of the catalyst is of unusual importance in these rhodium-catalyzed reactions.
It is also recognized in the art that there are certain reaction by-products which have an adverse effect on the rhodium catalyst. Specifically, it is taught in German Offenlegungsschrift No. 2062703 that a dimeric unsaturated aldehyde which is a by-product in producing butyraldehyde from propylene has an adverse effect on the rhodium catalyst if its concentration is allowed to build up to levels greater than about five weight percent in the liquid reaction medium. Otherwise, however, the same reference teaches that other high molecular weight by-products (in particular hydroxylic compounds) not only are not deleterious but in fact are very desirable as reaction solvents in the process. Similar teachings appear in British Pat. No. 1298331, which broadly recommends using the entirety of the reaction by-products as reaction solvent without ascribing deleterious properties to any of the components thereof.
Aside from the limited teachings regarding the desirability of avoiding an unlimited buildup of the unsaturated aldehyde by-products discussed above as they relate to deactivation of the rhodium catalyst, the existing prior art is largely silent on the somewhat-related matter of the loss of the triorganophosphorus ligand (e.g., triphenylphosphine) due to chemical combination with various other components of the reaction system. Yet loss of the ligand through, for example, the formation of inert high-boiling polymeric derivatives thereof, is in itself a significant cost item in the hydroformylation processes under consideration even though unit cost of the ligand species is very much lower than that of rhodium.
It is, accordingly, an object of the present invention to provide a method for reducing the rate at which the rhodium catalyst complex is deactivated during the course of the reaction of an olefin, carbon monoxide, and hydrogen in the presence of a catalyst comprising rhodium in complex combination with carbon monoxide and a triorganophosphorus ligand. It is another object to provide a method for reducing losses of the triorganophosphorus ligand due to formation of phosphorus-containing by-products during the course of the same hydroformylation reaction. It is a specific object to provide a method for reducing said catalyst deactivation and said loss of triorganophosphorus ligand in a process for converting an alkene to a carbonyl derivative thereof in the presence of a catalyst consisting essentially of rhodium in complex combination with carbon monoxide and triphenylphosphine. Other objects will be apparent from the following detailed description.